IMO, she is far too new to politics to be a credible VP nominee. The VP candidate has to be someone who is prepared to step in as President if that need ever arises. Senator McCain chose poorly sacrificing readiness to serve for garnering attention in his choosing Governor Palin. That choice marked what has been an occasional lack of discipline/erratic approach (often bundled into his persona as a "maverick") that also exhibited itself in his panicked response to the financial crisis e.g., canceling his debate appearance and then showing up when the debate itself was not postponed. In contrast, even as their ideologies are worlds apart, Cheney and Biden were safe picks in that either man could readily serve as President.
Wendy Davis has both tenacity and talent. Her rising to graduate from Harvard Law School with honors in spite of her having been a single mother--a very demanding task under very demanding circumstances--speaks volumes of her potential. Her tenacity and determination were on display in her filibuster that, at least, temporarily thwarted adoption of abortion-related legislation. She probably has the ability to serve effectively in a national capacity (appointment or Congress). That national service and her success in that role would be essential for her to become a credible VP possibility. There's no possible way she would really be prepared for the VP's office, much less Presidency, by the 2016 election. 2024, maybe even 2020, might be different. Of course, that demands of the job have not discouraged unprepared individuals from seeking the Presidency e.g. Herman Cain who held no elected office and seemed completely out of his league when it came to debating policy substance during his short-lived candidacy.
If Hillary Clinton seeks the Presidency, she will almost certainly make a reasonably safe pick for her running mate. In her service as Senator and later Secretary of State, she has not been a gambler so to speak. She is far more a manager/administrator than a revolutionary. A choice of an unseasoned running mate, even if that person were able to sustain a high public profile up to the 2016 race, would be completely out of character for her.